Reduction of the size and the inherent features of semiconductor devices (e.g., a metal-oxide semiconductor field-effect transistor) has enabled continued improvement in speed, performance, density, and cost per unit function of integrated circuits over the past few decades. In accordance with a design of the transistor and one of the inherent characteristics thereof, modulating the length of a channel region underlying a gate between a source and drain of the transistor alters a resistance associated with the channel region, thereby affecting performance of the transistor. More specifically, shortening the length of the channel region reduces a source-to-drain resistance of the transistor, which, assuming other parameters are maintained relatively constant, may allow an increase in current flow between the source and drain when a sufficient voltage is applied to the gate of the transistor.
To further enhance the performance of MOS devices, stress may be introduced in the channel region of a MOS transistor to improve carrier mobility. Generally, it is desirable to induce a tensile stress in the channel region of an n-type metal-oxide-semiconductor (“NMOS”) device in a source-to-drain direction, and to induce a compressive stress in the channel region of a p-type metal-oxide-semiconductor (“PMOS”) device in a source-to-drain direction.
A commonly used method for applying compressive stress to the channel regions of PMOS devices is growing SiGe stressors in the source and drain regions. Such a method typically includes the steps of forming a gate stack on a semiconductor substrate, forming spacers on sidewalls of the gate stack, forming recesses in the silicon substrate along gate spacers, epitaxially growing SiGe stressors in the recesses and annealing. Since SiGe has a greater lattice constant than silicon, it expands after annealing and applies a compressive stress to the channel region, which is located between a source SiGe stressor and a drain SiGe stressor.
The above-discussed method, however, suffers the drawback of pattern-loading effects, which occur due to a difference in pattern densities. The pattern-loading effects pertain to a phenomenon occurring upon simultaneous epitaxial growth in a region of a higher density and a region of a lower density. Due to a difference in growth rates of a film from one location to another, the amount of growth becomes locally dense or sparse depending on the local pattern density, and this causes non-uniformity in the thickness of the resulting film. Large variations in effective pattern density have been shown to result in significant and undesirable film thickness variation. For example, isolated active regions that are surrounded by regions having a large area ratio of dielectrics (meaning less surface area for the epitaxial growth) would have faster growth of the EPI layer than dense active regions. In addition, the composition of the EPI layer at the isolated active regions is also different from that of densely packed active regions. Particularly, this non-uniformity makes device formation process hard to control and device performance may be adversely affected.
The pattern-loading effects can be reduced by adjusting epitaxy parameters, such as reducing the process pressure or adjusting precursor flow rates. However, as a side effect, other epitaxy properties, such as composition, are also impacted by the changes of the pressure and gas flow rate. Additionally, the amount of reduction of the pattern-loading effects using this method is not satisfactory.
To effectively counteract the pattern-loading effect of SiGe growth, a layout design step known as a dummy pattern is used, wherein the circuit layout is modified and dummy patterns are added to locations with low pattern density. For selective epitaxial growth, dummy patterns are formed in sparse pattern regions over the dielectric material covering the regions. They are typically formed of materials similar to the material where growth is to occur. Selective epitaxial growth occurs on both desired regions and dummy patterns. The adding of dummy patterns helps in achieving more uniform pattern density across the wafer, thereby reducing pattern-loading effects. This method provides better results. However, additional process steps, hence higher costs, are involved. SiGe dummy patterns have to be formed in selective locations to make the density of the silicon patterns uniform.
Accordingly, what is needed in the art is a method for forming PMOS devices that may incorporate SiGe stressors to take advantage of the benefits associated with compressive stress while at the same time overcoming the deficiencies of the prior art.